1. Field of the Invention
This invention relates to novel fluoran compositions, to a process for preparing such compounds. Furthermore, the invention relates to pressure- or heat-sensitive recording sheets using such fluoran compounds, which are capable of providing black images having excellent stability, e.g., with respect to light.
The fluoran compounds of the present invention are particularly useful as dye precursors for use in recording materials such as pressure-sensitive recording sheets, heat-sensitive recording sheets, and energizable heat-sensitive recording sheets, and can also be applied to light-sensitive recording sheets, ultrasonic wave-recording sheets, electron beam-recording sheets, electrostatic recording sheets, light-sensitive printing plate materials, stamping materials, type ribbon, ink for ballpoint pen, crayon, and the like.
2. Description of the Prior Art
Recording systems utilizing a coloration reaction caused by contacting an almost colorless electron-donating compound with an almost colorless electron-accepting compound have long been known, particularly systems using pressure-sensitive recording sheets and heat-sensitive recording sheets.
In general, the pressure-sensitive recording sheet comprises a set of sheets: (1) an upper sheet having coated on the support thereof microcapsules prepared by dissolving an electron-donating colorless dye (hereinafter referred to as a color former) in a suitable solvent, emulsifying it into particles of a several-micron size, and encapsulating the droplets with a high molecular weight compound such as gelatin; and (2) a lower sheet comprising a support having coated thereon an electron-accepting compound (hereinafter referred to as a color developer). Recording is achieved by applying writing pressure or impact pressure to the sheets, arranged with the coated sides facing each other; the microcapsules are thereby ruptured, and release and transfer the color former to the color developer-coated surface, thereby causing a coloration reaction. Detailed descriptions of such pressure-sensitive recording sheets are found, for example, in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,730,457 and 3,418,250.
On the other hand, the most popular type of heat-sensitive recording sheet comprises a support having provided thereon a color former and a color developer carried in a binder so as not to come prematurely into contact with each other. Upon being heated, at least one of the color developer and the color former is melted, and comes into contact with the other, thereby causing a coloration reaction, thus recording color in heated areas of said sheet. Detailed descriptions of such heat-sensitive sheets are found, for example, in Japanese Patent Publication Nos. 4160/68, 3680/69 and in U.S. Pat. No. 2,939,009.
The pressure-sensitive and heat-sensitive recording sheets containing combinations of color former and color developer can provide various colored images, depending on selection of appropriate kinds of color former and color developer. It has recently been desired to prepare pressure- and heat-sensitive recording sheets capable of providing black images, in order that further copies of the recording sheets can be obtained. In principle, a black color image can be obtained by suitably mixing several kinds of color formers forming different colors. However, since the coloration rate and/or fastness against light, temperature, and humidity varies depending upon the particular color former used, the resulting color using such a combination changes after recording with time and/or under certain storage conditions. In addition, the use of many color formers complicates the production steps. Therefore, investigations have been made to obtain a black color image by using a single color former. However, there has not yet been discovered any black color former that is satisfactory in all respects, including hue, fastness, cost, etc. Thus, conventional black color-forming pressure- and heat-sensitive recording sheets have not been totally satisfactory.
Heretofore, known fluoran compounds have been used as dye precursors for use in recording materials in U.S. Pat. Nos. 3,501,331, 4,007,195, 4,024,157 and 3,920,510. However, conventional fluoran compounds have defects in that the compounds are unstable in the atmosphere, and dyes formed therefrom have poor fastness against light, humidity, etc.